I don't know how much you can trust trainer power curves from the Web. For my Cycleops MAG+, web curves are off literally by a factor of 2 (the trainer eats 200 watt at 20 mph at the highest resistance setting, as measured by the power meter; the only available web curve claims that it should eat 400 watt.)
From general principles, viscosity of the fluid in a fluid trainer drops off rather dramatically with temperature (if it's anything like typical motor oil, viscosity would drop in half for every 30 F), and resistance at constant speed would drop as well as the fluid heats up. So, resistance would vary depending on what you did during the workout, and even on the little things like room temperature and having or not having a fan near the trainer. A short interval at 21 mph with cool fluid would present more resistance than sustained 21 mph. Motor oil also has substantial specific heat capacity: assuming there's ~1 liter of oil in the trainer (complete guess) and there's no heat loss, it takes 1.5 minutes at 300 W to heat the oil by 30 F.